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Myasthenia Gravis and Autoantibodies

Myasthenia Gravis and Autoantibodies

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Myasthenia Gravis and Autoantibodies

Clinical Background

Myasthenia gravis (MG) is an autoimmune disease characterized by skeletal muscle weakness due to defective neuromuscular transmission. Onset may be gradual or acute following viral infection or pregnancy. Treatment options include anticholinesterase drugs, thymectomy, corticosteroids, plasmapheresis, and immunosuppressive therapy.

The majority of MG patients demonstrate elevated serum levels of acetylcholine receptor (AChR) antibodies. These antibodies cause a loss of receptors, partial or complete inhibition of receptor function, and complement-mediated focal lysis of the post-synaptic membrane. Three types of antibodies may be involved: binding, blocking, and modulating. Binding antibodies are detected in 69% to 82% of MG patients with generalized disease and 59% of MG patients with only ocular muscle involvement. Modulating antibodies are found at approximately the same frequency as binding antibodies. However, approximately 8% of patients have positive results for only 1 of the 2 tests. Modulating antibodies are found in the absence of binding antibodies in 4% of patients. Blocking antibodies, which have a similar sensitivity as modulating antibodies for MG diagnosis, are detectable in about 52% of patients with generalized disease and about 30% of patients with ocular MG. Fewer than 1% of patients have blocking antibodies without binding antibodies, and blocking antibodies are rare in non-MG disease. Therefore, blocking antibody testing may help identify false-negative binding antibody test results.

Other types of antibodies such as striational and muscle-specific receptor tyrosine kinase (MuSK) antibodies are also useful in MG diagnosis. Striational antibodies are found in 30% of all MG patients, in 50% of MG patients with late-onset disease, and in 95% of those with thymomas. These autoantibodies recognize epitopes on skeletal muscle proteins including myosin, actin, actinin, filamin and titin. Some types of striational antibodies, including those to titin, can provide more clinical information and aid in the diagnosis of thymoma. MuSK antibodies may be useful for MG diagnosis in patients who test negative for AChR antibodies. Patients with antibodies to MuSK are much less likely to have a thymoma. Guidelines for ordering AChR, striational, titin, and MuSK antibody tests can be found in the Test Selection Guide below.

Clinical Utility of Laboratory Tests

  • Differential diagnosis of MG-like muscle weakness

  • Monitoring therapeutic response

  • Differential diagnosis of thymoma

Test Selection Guide

Applications AChR Antibody Striational Antibody Titin Antibody

MuSK

Antibody

Binding Blocking Modulating
Initial screening x x x
Confirmation of diagnosis x xa x x xa
Monitoring therapeutic response and disease progression x x x x
Thymoma diagnosis   x x  

This table is provided for informational purposes only and is not intended as medical advice. A physician's test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.

AChR, acetylcholine receptor.

a When binding antibody is negative.

Method

AChR Antibodies

Binding Antibody (Test Code 206)

Patient specimens and calibrators are incubated with detergent-solubilized, fetal and adult AChR that are labeled with 125I-alpha-bungarotoxin. The resulting labeled AChR/autoantibody complex is then precipitated with anti-human IgG. The amount of radioactivity in the precipitate is directly proportional to the amount of antibody present.

Blocking Antibody (Test Code 34459)

Patient specimens and calibrators are incubated with detergent-solubilized fetal and adult AChR and 125I-alpha-bungarotoxin. Acetylcholine blocking antibody in the patient sample competes with the 125I-alpha-bungarotoxin for a limited number of binding sites on the receptor. The resulting AchR/blocking antibody complexes are then precipitated with concanavalin-A Sepharose®. The amount of radioactivity in the precipitate is inversely proportional to the amount of antibody present.

Modulating Antibody (Test Code 26474)

This radiobinding assay employs muscle-type AChR in live human sarcoma cells (TE671). Modulating antibodies in the patient serum bind to the receptors; any remaining non-modulated acetylcholine receptor binding sites are then bound by radiolabeled bungarotoxin. The bound fraction is separated from the free radiolabeled bungarotoxin by repeated washing and is then quantified. Binding specificity is determined by pre-addition of the carbamylcholine agonist in parallel cultures. Note that this test detects both blocking and modulating antibodies.

Striated [Striational] Muscle Antibody (Test Code 266)

Acetone-fixed, longitudinal sections of monkey skeletal muscle are placed in a slide well and patient serum is overlaid. Striated muscle antibodies in the patient serum bind to muscle antigens and are detected by fluorescence microscopy using a FITC-labeled, primate-absorbed, anti-human IgG and an Evans Blue counterstain. The antibody titer is defined as the highest dilution that produces a 1+ fluorescence intensity.

Titin Autoantibody (Test Code 1480*)

This enzyme-linked immunosorbent assay (ELISA) detects titin antibody in patient serum. Results are reported as negative, borderline, or positive.

MuSK Antibody (Test Code 18842/482*)

This radioimmunoassay determines the presence or absence of MuSK antibody. A titer of 1:10 or greater is considered positive for antibody presence.

*Test code 1480 is available from Athena Diagnostics, a Quest Diagnostics company. The MuSK antibody test is performed at Athena Diagnostics. It can be ordered directly from Athena using test code 482 or from Quest Diagnostics using test code 18842.

Interpretive Information

Elevated levels of AChR antibodies in patients with appropriate symptoms are diagnostic of MG; higher levels are associated with more severe disease. Husain et al found that binding antibodies were present in 82% of patients with moderate/severe generalized disease; 69% of patients with mild, generalized disease; and 59% of patients with ocular myasthenia. In patients who test negative for binding antibodies, the presence of modulating or blocking antibodies may indicate MG.

The presence of MuSK antibodies is associated with AChR antibody-negative MG.

The presence of striational antibodies, including antibodies to titin, is associated with MG comorbid with thymomas and with late-onset MG.

References

  1. Aarli JA, Lefvert AK, Tonder O. Thymoma specific antibodies in sera from patients with myasthenia gravis demonstrated by indirect haemagglutination. J Neuroimm. 1981;1:421–427.

  2. Conroy WG, Saedis MS, Lindstrom J. TE671 cells express an abundance of a partially mature acetylcholine receptor α subunit which has characteristics of an assembly intermediate. J Biol Chem. 1990;265:21642-21651.

  3. Drachman DB, Adams RN, Josifek LF, et al. Functional activities of autoantibodies to acetylcholine receptors and the clinical severity of myasthenia gravis. N Engl J Med. 1982;307:769-775.

  4. Howard FM Jr, Lennon VA, Finley J, et al. Clinical correlations of antibodies that bind, block, or modulate human acetylcholine receptors in myasthenia gravis. Ann N Y Acad Sci. 1987;505:526-538.

  5. Husain AM, Massey JM, Howard JF, et al. Acetylcholine receptor antibody measurements in acquired myasthenia gravis. Diagnostic sensitivity and predictive value for thymoma. Ann N Y Acad Sci. 1998;841:471-474.

  6. Juel VC, Massey JM. Myasthenia gravis. Orphanet J Rare Dis. 2007;2:44.

  7. Lennon VA, Jones G, Howard F, Elveback L. Autoantibodies to acetylcholine receptors in myasthenia gravis (letter). N Engl J Med. 1983;308:402-403.

  8. Levinson AI and Wheatley LM. Myasthenia gravis. In Clinical Immunology Principles and Practice. Rich R (ed), St. Louis, MO: Mosby Publishers, 1996.

  9. Mittag TW. Antibodies to receptor proteins: the significance of nicotinic acetylcholine receptor antibodies. J Clin Immunoassay. 1983;6:S25-S32.

  10. Ohta M, Ohta K, Itoh N, Kurobe M, Hayashi K, Nishitani H. Anti-skeletal muscle antibodies in the sera from myasthenic patients with thymoma: identification of antimyosin, actomysin, actin, and α-actinin antibodies by a solidphase radioimmunoassay and a Western blotting analysis. Clin Chim Acta. 1990;187:255–264.

  11. Pachner AR. Anti-acetylcholine receptor antibodies block bungarotoxin binding to native human acetylcholine receptor on the surface of TE671 cells. Neurology. 1989;39:1057-1061.

  12. Penn AS, Richman DP, Ruff RL, et al, eds. Myasthenia gravis and related disorders: experimental and clinical aspects. Conference proceedings. Washington, DC, April 12-15, 1992. Ann N Y Acad Sci. 1993;681:1-622.

  13. Romi F. Thymoma in myasthenia gravis: from diagnosis to treatment. Autoimmune Dis. 2011; 2011:474512.

  14. Suzuki S, Utsugisawa K, Nagane Y, Suzuki N. Three types of striational antibodies in myasthenia gravis. Autoimmune Diseases. 2011; doi:10.4061/2011/740583.

  15. Vincent A. Acetylcholine receptor autoantibodies. In: Peter JB, Shoenfeld Y, eds. Autoantibodies. Amsterdam: Elsevier; 1996:1-9.

  16. Williams CL, Lennon VA. Thymic B lymphocyte clones from patients with myasthenia gravis secrete monoclonal striational autoantibodies reacting with myosin, α actinin, or actin. J Exp Med. 1986;164:1043–1059.

  17. Yamamoto T, Sato T, Sugita H. Antifilamin, antivinculin, and antitropomyosin antibodies in myasthenia gravis. Neurology. 1987; 37:1329–1333.
     

The modulating antibody test was developed and its performance characteristics have been determined by Quest Diagnostics Nichols Institute, San Juan Capistrano. Performance characteristics refer to the analytical performance of the test.

The striated [striational] muscle antibody test was developed and its performance characteristics have been determined by Quest Diagnostics Nichols Institute. They have not been cleared or approved by the U.S. Food and Drug Administration. The FDA has determined that such clearance or approval is not necessary. Performance characteristics refer to the analytical performance of the tests.

Content reviewed 06/2015

 
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* The tests listed by specialist are a select group of tests offered. For a complete list of Quest Diagnostics tests, please refer to our Directory of Services.